Hr1203 d4 diode functionality


Please let me know the function of the D4 diode in the below datasheet application circuit.


By lookin

Prathuesh K

Hi Prathuesh,

Thank you for contacting MPS. Let me get our expert to comment on the function of that diode.

Hi Prathuesh:

The diode D4’s function is to pre-charge the Cbus cap that is the Bulk storage cap across the Vbus. This is done as soon as AC is applied and before the Boost stage is enabled and starts switching. This reduces the peak flux in the Boost Inductor and the current stress in the inductor and the Boost FET at start up. Without this pre-charge, during the first few cycles, the reset volt-seconds ( during OFF time) across the inductor will be zero or very low. This reset takes place during in the OFF time of the Boost FET . As a result the volt seconds across the inductor will not balance and therefore can cause saturation of the inductor at start up.


1 Like

Another consideration is that some of the fast recovery boost diodes are not able to tolerate the high inrush currents that would otherwise flow through them if D4 were not in place. Specifically some of the SiC diodes are quite fragile in this regard.

1 Like

Thank you so much for the detail explanation.